A Twitter user’s @-mentions: comments in response to their posts, or other discussion threads they have been tagged in. When a user is mentioned, they are usually notified, which can turn out bad.
When a user replies to a
post, the author of the original
post is “mentioned”. If the author’s post is controversial or blows up for some reason, they
will get lots of repliers in their menchies. This often makes their notifications almost
useless, since they’re flooded by responses to
one post.
Other times,
two or more repliers get into a debate in response to a different user’s original post, littering the OP’s menchies with irrelevant chatter. For obvious reasons, it sucks to get a bunch of notifications on an argument you have no interest in.